Bone health is a significant public health issue in the United States. An estimated 10 million Americans age 50 and older are living with osteoporosis, and approximately 50% of women and 25% of men over age 50 will experience an osteoporosis-related fracture in their remaining lifetime. Recent research advancements have produced effective measures to improve and maintain bone health, including exercise, diet, screening tests, pharmacotherapies, and fall prevention strategies. These findings, however, have not been fully incorporated into the daily lives of adults, and many are unaware of the magnitude of bone-health problems. Only 11-44% of adults at high risk for osteoporosis take calcium and vitamin D supplements, and one third of women over age 55 have not discussed bone health issues with their care providers. Greater efforts must be made to effectively disseminate evidence-based research findings to improve bone health behaviors of the public. The Internet, with its increasing popularity, can be an effective tool in this endeavor. Despite a great deal of available online health resources, there has been a lack of research investigating effective methods to package and deliver these resources to yield positive public health outcomes. Our prior R21 grant project showed some preliminary effectiveness of using a social cognitive theory (SCT)-based online health promotion intervention. Expanding upon our prior findings and using an innovative approach combining two models of the SCT and the RE-AIM framework (Reach, Effectiveness, Adoption, Implementation, Maintenance), in this proposal, we propose a large scale online study (N = 768) that will compare the long-term impact of two SCT-based online bone health interventions in adults age 50 and older targeting two large online communities (SeniorNet, MyHealtheVet). The two interventions are (1) an 8-week SCT-based Online Bone Health (TO-BoneHealth) program and (2) a 12-month TO-BoneHealth Plus program, including the TO-BoneHealth program followed by biweekly theory- based eNewsletters with follow-up of each individual's bone health behaviors (booster intervention) for 10 months. The interventions' impact will be assessed on the following dimensions of the modified RE-AIM framework: (a) Effectiveness (knowledge, selected bone health behaviors, fall incidence, initiation of a discussion about bone health with a primary care provider, eHealth literacy); (b) Reach (number of participants the program reached); (c) Implementation (program usage); and (d) Maintenance (participants' bone health behavior maintenance). The Effectiveness dimension will be assessed using a randomized controlled trial employing a 3-group design (2 intervention and 1 control groups) with repeated measures (baseline, 8 weeks, 6, 12, and 18 months). The primary analytic approach for the Effectiveness dimension will involve mixed linear models analyses. Other impact dimensions will be assessed using descriptive statistics. With the rapid growth of online communities, findings from this study will significantly contribute to current eHealth practice and research and serve as a dissemination model for other health promotion projects targeting online communities. PUBLIC HEALTH RELEVANCE: Bone health is a significant public health issue in the United States, with approximately 10 million Americans age 50 and older living with osteoporosis. Several effective measures have been found to improve and maintain bone health. Many individuals, however, are unaware of the significance of bone health problems and do not follow recommended guidelines for bone health behaviors. Maximizing the popularity of the Internet, this innovative study will examine the impact of two online motivational health promotion interventions on improving mature adults' bone health behaviors. With the rapid growth of active online communities, findings from the proposed study will significantly contribute to public health by advancing current face-to-face health promotion efforts to online settings serving as a model approach.